Thin films are useful for a wide range of optical applications, such as antireflective (“AR”) coatings, high-reflective (“HR”) coatings, dielectric mirrors, thin film interference filters, active light emitters, gratings, and color generators, to name a few examples. Compact size and environmental stability are two properties of optical thin films that encourage their deployment in modern applications including optical communications, lighting, vision, instrumentation, medical devices, computer monitors, display systems, etc. Certain types of optical thin films manipulate light by interference, which entails an additive or subtractive process in which the amplitudes of two or more overlapping light waves systematically attenuate or reinforce one another. Interference can produce polarization, wavelength-selective transmission and/or reflection, beam splitting, or various other effects on a light beam, according to the design of the thin film and its interaction with adjacent elements in an environment of an optical system.
Thin film interference filters typically comprise a stack of thin film layers or a plurality of laminates that collaboratively provide a band, span, or range of color transmission and another band, span, or range of color reflection. Such thin film interference filters often provide a pass band that is bracketed by two bands of reflection. That is, a spectral or color region of high transmission (and low reflectivity) lies between two spectral or color regions of low transmission (and high reflectivity). Many conventional interference filters are better suited to providing a pass band or a narrow spectral band of high transparency than to providing a stop band or a narrow spectral band of high reflectivity, sometimes referred to as a notch.
FIG. 3A illustrates a cross sectional view of a portion 325 of a conventional thin film apparatus, for example a band pass filter, comprising a first layer of optical material 360 adhering or laminated to a second layer of optical material 370. The accompanying FIG. 3B graphically depicts a representative refractive index profile 300 for the materials 360, 370 illustrated in FIG. 3B.
The optical materials 360, 370 typically have different refractive indices 310, 320, for example one being relatively high and one being relatively low. The material 370 might be silicon dioxide (SiO2), of relatively low refractive index 320, while the material 360 might be tantalum pentoxide (Ta2O5), of relatively high refractive index 310. In the conventional approach, each material layer 360, 370 typically has a thickness at least on the order of one-fourth of the wavelength of the light that the system 325 handles.
The interface 340 lies between two materials 360, 370, with FIG. 3 illustrating two more interfaces 330, 350. As shown in FIG. 3B, the refractive index typically changes abruptly at the boundaries or interfaces 330, 340, 350 between each layer of the material 360 and the material 370. Thus, the system 325 typically has a crisp change in the material composition between the two material regions 360, 370.
The refractive index change between the two materials 360, 370, at the interface 340, can usefully induce a light reflection that, when combined with reflections from other interfaces 330, 350, produces optical interference. However in some applications, a more gradually change in refractive index at the interfaces 330, 340, 350 would be desirable. For example, thin film notch filters that produce a narrow spectral band of reflection between two spectral regions of transmission may benefit from having gradual changes in refractive index at layer boundaries.
A class of notch filters known as “rugate” filters typically use a conventional approach to providing a gradual refractive index change at a filter's thin film layer interfaces. In a rugate filter, each interface between adjoining thin film layers typically comprises a blended combination of the materials of the two adjoining layers. That is, if the apparatus 325 illustrated in FIG. 3A was a rugate filter, the interface 340 would have a composition that gradually changed between the material 370 and the material 360 (along the Z axis).
Rugate filters are typically fabricated in a vacuum chamber via thin film deposition. A source in the chamber outputs particles of high-refractive index material that accumulate to create the high-refractive index layers. Another source outputs particles of low-refractive index materials to create the adjoining low-refractive index layers. When forming the rugate's blended interface, both the high-refractive index source and the low-refractive index source may actively output their respective materials. After forming the major portion of the high-refractive index layer, the high-refractive index source gradually reduces its rate of outputting high-refractive index particles. As the deposition rate of high-refractive index particles decreases, the low-refractive index source begins outputting low-refractive index particles and gradually increases the deposition rate of low-refractive index particles. Accordingly, the rugate's blended interface can be formed by simultaneously depositing high- and low-index materials at controlled deposition rates.
However, with conventional technologies, providing a sufficient level of control of the deposition rates can be difficult. If the relative deposition rates of the high- and low-index materials are not precisely controlled, the rugate's blended layer interfaces may fail to provide the desired optical properties. Another problem with many conventional techniques for producing rugate filters can occur with the material properties that result from the blended composition itself. Two materials that are individually well suited to forming pure layers may not be compatible with one another when mixed. That is, although two pure material layers may adhere to one another, those two materials may not form a stable or robust structure when blended or when mixed at the atomic, molecular, or particulate level. For example, the blended composition may have thermal expansion properties or sensitivities that are less desirable than the corresponding properties of unblended layers. Further, processing the appropriate materials in a manner that facilitates successful blending can be problematic.
To address these representative deficiencies in the art, what is needed is an improved capability for managing light propagating near an interface between two optical materials or media. A further need exists for a structure that can provide a smooth or gradual refractive index transition between two materials. Yet another need exists for a system that can provide a smooth material transition at an interface between two sections of distinct optical materials. Still another need exists for an efficient or robust process to fabricate thin film devices in a manner that provides desirable interfaces between adjoining film layers. One more need exists for a notch filter that offers a high level of optical performance, that provides a low level of environmental and/or thermal sensitivity, and that can be cost-effectively manufactured. Finally, a need exists for a process of forming filters with rugate-type optical characteristics without necessarily blending optical materials in a deposition chamber. A technology filling one or more of these needs would enhance the precision with which optical thin films manipulate light and would facilitate cost-effective utilization of optical thin films in numerous applications.